A cellular wireless network can comprise a plurality of cells, wherein each cell comprises a base station. Whereas in some cellular wireless networks the cells are of similar size, other cellular networks can comprise cells of different sizes including macrocells, microcells as well as picocells. The use of MIMO (Multiple Input-Multiple Output) antenna techniques has increased the possible data throughput of a cellular network. In a MIMO system there are more than one transmit antenna to send a signal on the same frequency and more than one receive antenna. Whereas traditional cellular networks generally provide a best service under line of sight conditions, the MIMO system is most sufficient under rich scattering conditions where signals bounce around the environment. Under rich scattering conditions signals from different antennas take multiple paths to reach the UE at different times. To achieve a high throughput the MIMO system uses a technique, the so-called spatial multiplexing. By use of spatial multiplexing the data rate can be increased. To do this, the data is divided into separate streams, wherein the data streams are transmitted independently via separate antennas. In spatial multiplexing each antenna has a different data stream to multiple receiving antennas. These data streams are then reconstructed separately by the UE. In spatial multiplexing, although multiple data streams are transmitted, the total power of the transmission remains the same. With spatial multiplexing the total signal to noise ratio, SNR, is distributed between multiple data streams, wherein each of which has a lower power level. Consequently, each data stream contains a lower SNR than would be possible with a single data stream.
Each set of data sent through the antennas in spatial multiplexing operation is also called a layer. In spatial multiplexing, rank refers to the number of data streams transmitted over the same kind of frequency resource, corresponding to the number of layers.
FIG. 1 shows a diagram for illustrating spatial multiplexing as employed in a conventional wireless cellular network. Spatial multiplexing works by creating separate data streams on multiple antennas. With spatial multiplexing, independent data streams can be transmitted simultaneously on the same frequency resource by mapping them to so called spatial layers. The number of spatial layers is the same as the rank, R, of the precoding matrix used for data transmissions.
As shown in FIG. 1 in a multi layer transmission, data arriving from a higher level process comprises codewords. Each codeword is then mapped onto one or more layers. Each layer is then mapped onto one or more antennas using a precoding matrix.
In the present patent application, the following abbreviations are used:
ACI Active Codebook Information
BBU Base Band Unit
CAS Central Antenna System
CQI Channel Quality Indicator
CPM Cluster Precoding Matrix
CB Codebook
DAS Distributed Antenna System
ICI Inter Cell Interference
MIMO Multiple Input Multiple Output
PDCCH Physical Downlink Control Channel
PM Precoding Matrix
PMI Precoding Matrix Indicator
RI Rank Indicator
RRU Remote Radio Unit
SINR Signal to Noise plus Interference Ratio
UE User Equipment
A cellular network can operate in an open or closed loop. In an open loop operation the base station receives minimal information.
In closed loop operations the UE analyses the channel conditions of each transmitting antenna including the multi-path conditions. In closed loop operations the UE provides an RI as well as a PMI, which indicates the optimum PM for the current channel conditions. Moreover, the UE can provide a CQI given the RI and the PMI. This allows the base station to quickly and effectively adapt the transmission to channel conditions. Closed loop operations are particularly relevant for spatial multiplexing, where the MIMO system offers the greatest throughput gains.
In multiple layer transmissions data arrives from higher level processes in one or more codewords, wherein each codeword is then mapped onto one or more layers. Each layer is then mapped onto one or more antennas using a PM. This PM can be taken from a CB, wherein a predetermined set of PMs is stored. Each PM comprises a corresponding PMI.
The MIMO system can be implemented for instance in an LTE network. In general, an SINR of a UE depends on both signal power and ICI. The prediction or estimation of ICI is an important factor to improve the CQI precision and thus link adaption. In real systems, link adaption is always affected by a delay of the CQI feedback. The effect of Doppler shift may be alleviated by channel prediction methods.
In a standard wireless network such as a LTE network each UE provides feedback on the PMI which can be wideband, and CQI which can be wideband or provided per each subband.
Based on the received feedback, a base station scheduler decides to provide resources in the time/frequency/space for a set of UEs in the respective cell of the wireless network. Each base station can select its UEs according to the following predetermined scheduling policies on the basis of the received feedback. The main reasons for performance loss are related to a feedback delay Channel conditions may change in the meantime and therefore scheduling decisions may not be optimal any longer. This cell-dependent CB selection for a PM can be used for a precoding downlink transmission. Each cell of the wireless network may optimize the downlink precoding configuration for its own UEs. Hence, after reconfiguration, the ICI which is caused by neighbouring cells can significantly change its spatial structure. Thus, the downlink interference observed at the UE side may be highly dynamic.
The experienced multicell or ICI strongly depends on the PMs selected by the neighbouring base stations. When base stations adjust the PMs due to user movements or altered traffic conditions, the ICI experienced by UE in the neighbouring cells may significantly change. Precoding and transmission configurations in these cells of the wireless network, which were based on the interference situation generated by the previous selection of PMs and precoding weights in the cell, do then no longer match the actual interference situation and a performance degradation can be observed.
Accordingly, there is a need to provide an apparatus and a method for providing a reliable estimation of ICI within a wireless network using changing PMs.